Electrical Panel Construction
Electrical panels are used to distribute electrical energy within buildings. Depending on the voltage, purpose, and configuration, electrical panels may be described as “load centers,” “panelboards,” “distribution boards,” “switchboards,” “switchgear,” or by other terms used in the industry and known to those skilled in the art. For the purposes of this disclosure, an electrical panel includes a metallic enclosure, one or more incoming power circuits (feeders or electrical power sources), one or more outgoing power circuits (serving electrical loads), and one or more circuit-switching devices which control the flow of electricity among the circuits.
Inside the metallic enclosure of the electrical panel, the incoming circuits may be connected to the circuit breakers directly or via an electrical bus. Connection of the incoming circuit via an electrical bus is the most common configuration and will be assumed, without limitation, throughout the remainder of this application. Most applications will include two or three energized (“hot” or “live”) buses of different polarities and one or two neutral buses. The energized buses are normally of two or three different phases. Neutral buses are normally bonded to ground potential at the building's electrical service entrance and allowed to electrically “float” above ground potential throughout the rest of the building. In demanding applications, for example above 1000 volts, the electrical panel may also include a separate ground bus.
The circuit-switching devices are most commonly circuit breakers. Therefore reference to “circuit breakers” throughout this application includes, without limitation, other circuit-switching devices, including switches, fuses, and switch-fuse combinations. The circuit breakers have controls which are electrically insulated from the incoming and outgoing power circuits. Each load circuit is connected to the bus through a circuit breaker. The circuit breakers allow the load circuits to be manually de-energized using the insulated control of the circuit breaker. The circuit breaker also automatically de-energizes the circuit in response to an overload or short circuit. Depending on the application, a main circuit breaker (not shown) may be provided to isolate the buses from the incoming circuits. A main circuit breaker will often be required by code when the number of load circuits exceeds a certain number.
FIG. 1 illustrates the circuit diagram of an exemplary electrical panel. In this example, the electrical panel has two incoming circuits 100 (two-phase) and twelve outgoing circuits 102. Each outgoing circuit 102 is connected to a circuit breaker 104. Each circuit breaker 104 is connected in turn to an energized bus 106. In this example, the neutral bus 108 is bonded 110 to ground. The energized buses 106, neutral bus 108, and circuit breakers 104 are all contained within a grounded metallic enclosure 114. FIG. 2 illustrates the circuit diagram for a similar exemplary electrical panel, in this case having three incoming circuits (three-phase).
FIG. 3 illustrates the exterior of an electrical panel 300. The metallic enclosure 114 is electrically grounded. The metallic enclosure 114 normally includes a lockable door 302 and a removable front panel 304. When locked, the door 302 prevents unauthorized access to the insulated controls 306 of the circuit breakers 104. When locked, cover also prevents removal of the front panel 304. The front panel 304 is held in place by threaded fasteners 310.
The insulated controls 306 of the circuit breakers 104 extend through at least one wall of the metallic enclosure 114, normally the front panel 304. This allows a user to operate the circuit breaker(s) without exposure to any energized parts within the metallic enclosure 114. Common types of insulated controls 306 include switches, levers, and push-buttons, although other types of controls may be used. In some configurations the removable front panel 304 may have permanently removable sections 308 to accommodate adding circuit breakers 104 to the electrical panel 300. These removable sections 308 are called “knockouts” because they are removed by breaking a weld or a perforation in the metallic panel, usually with a hammer, screwdriver, and pliers. If a knockout 308 is removed and no corresponding circuit breaker is added to the electrical panel, the energized bus will be exposed in the empty breaker position. Special inserts, called “blanks,” are available which snap into the front panel and prevent the energized bus from being exposed.
The International Electrotechnical Commission (IEC) publishes standards for controlling access to electrical equipment. These Ingress Protection (IP) standards consist of a two-digit number. The first digit defines the protection against the ingress of solid objects. The second digit defines the protection against the ingress of liquids. For example, an IP31 enclosure provides protection against the ingress of solid objects 2.5 mm in diameter and against water dripping from above. In the United States, the National Electrical Manufacturer's Association (NEMA) produces a similar standard for ingress protection. The IP ratings are summarized in the table below:
TABLE 1Solid Object ProtectionObject Liquid ProtectionsizeObject sizeprotectedprotected LevelagainstEffective againstLevelagainst0NotNo protection against 0Not protectedprotectedcontact and ingress of objects1>50 mmAny large surface 1Dripping of the body, such aswaterthe back of the hand,but no protectionagainst deliberate contact with a body part.2>12.5 mmFingers or similar 2Dripping objects.water whentilted up to 15°3>2.5 mmTools, thick wires, etc.3Spraying water4>1 mmMost wires, screws, etc.4Splashing water5DustIngress of dust is not 5Water jetsProtectedentirely prevented,but it must not enterin sufficient quantityto interfere with thesatisfactory operation of the equipment; complete protection against contact.6Dust No ingress of dust; 6Powerful Tightcomplete protectionwater jetsagainst contact.7Immersion up to 1 m8Immersion beyond 1 mIndustry Work Practices
When working on the circuits in the electrical panel 300, the front panel 304 of the metallic enclosure 114 must be removed to expose the clamped and/or bolted connections of each circuit to the bus or circuit breakers. This work may include, but is not limited to: testing whether a circuit is energized, troubleshooting a circuit, adding a circuit, removing a circuit, upgrading a circuit, moving a circuit to a different circuit breaker, replacing a faulty circuit breaker, and upgrading a circuit breaker. In short, any work involving testing, moving a circuit breaker 104, or moving a wire will likely require removing the front panel 304 of the metallic enclosure 114.
If any of the incoming circuits are energized, removing the front panel will expose the worker to energized parts. Exposure to energized parts creates a hazard of electrocution and a hazard of an arc flash. An arc flash occurs when electricity “jumps” through the air from an energized conductor to a grounded conductor or a conductor of a different polarity. Air is normally an insulator, so the electricity must ionize the air, releasing light and heat. In some cases, the energy released is enough to create an explosive pressure wave and eject electrical components and molten metal. While the risk of electrocution is based primarily on the nominal system voltage, the hazard from an arc flash is based on both the system voltage and the short-circuit current. Unlike voltage, available short-circuit current must be determined on a panel-by-panel basis by an engineering study, making it much more difficult to classify the arc-flash hazard.
In the past, work safety practices focused only on reducing the risk of electrocution. Workers could work on energized parts if they used appropriate Personal Protective Equipment (PPE). For example, work on energized parts required special electrically insulated tools and/or specially-rated electrically insulated gloves. Rules may also have required hard hats or boots specially rated for their electrically insulating properties.
Another work safety practice is called “lockout/tagout.” This practice is intended to prevent accidents from working on an energized circuit in the mistaken belief that the circuit is de-energized. When a circuit breaker is opened, de-energizing the circuit, lockout/tagout requires attaching a lock and/or tag to the insulated control of the circuit breaker. The lock and/or tag prevents accidentally closing the circuit breaker and energizing the circuit, which could endanger an unsuspecting person working elsewhere on that circuit. In many circumstances additional devices may be temporarily fitted to the insulated control of the circuit breaker to accommodate the lock or tag.
As industry awareness of the arc-flash hazard grew, the Occupational Health and Safety Administration (OSHA), a federal agency governing workplace safety, created new rules to reduce the risk of arc-flash injuries. The new rules required that “[l]ive parts [above 50 volts] to which an employee may be exposed shall be de-energized before the employee works on or near them, unless the employer can demonstrate that de-energizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations.” 29 CFR 1910.333(a)(1). The old practice of working on or near exposed energized circuits, even with appropriate PPE, is thus prohibited unless the employer proves that de-energizing all of the circuits increases hazards or is “infeasible”.
Energized work is allowed to avoid introducing “additional or increased hazards” only when life-safety equipment is de-energized. For example, circuits supplying fire alarms, sprinkler pumps, and life-support equipment may be worked-on while energized. This limitation is strictly enforced.
Testing and troubleshooting de-energized circuits is considered “infeasible” because electrical power is needed to check an electrical circuit's function. Therefore, testing of energized circuits is allowed, but no other work may be done on or near them. The use of “tools” (i.e. metallic tools which are likely to cause a flash-over, even if they insulate the user from electrocution) is specifically prohibited.